1. Field of the Invention
This invention related to friction stir welding and more particularly to a method for insuring 100% penetration of the material thickness.
2. Description of the Related Art
The Welding Institute in Cambridge, England has developed a new method of joining materials by fiction stir welding (see U.S. Pat. No. 5,460,317). This method employs a tool having a pin which is plunged into and stirs the material to be joined to a plastic state. The pin preferably has a configuration for forcing the plasticized material downward and backward. When the pin is moved along the faying surface the plasticized material flows from the front of the pin to the rear as the pin translates the faying surface. A shoulder at the top of the pin keeps plasticized material from leaving the joint region. The pin extends almost to the bottom of the material to be joined. The pin can not extend all the way through the material since the material to be joined is resting on a back up bar. When the plasticized material is forced downward it is resisted by the material below the tip of the pin sometimes resulting in a lack of penetration all the way to the bottom of the faying surface and also by the back up bar at the bottom of the faying surfaces. The incomplete penetration of the weld can not be detected because the pin leaves an untouched faying surface portion and because of the intimate contact of the back up plate with the faying surface. X-rays and other means of detecting the weld quality are expensive, difficult to use and unreliable, especially to detect lack of penetration.